Exhaust gas purification apparatus for internal combustion engine

ABSTRACT

An exhaust gas purification apparatus includes a venturi constituting part of an exhaust pipe and formed by joining two axially divided parts together, and a reducing agent supply device for injecting an aqueous urea solution into the exhaust pipe. The venturi is positioned such that joints thereof are located to the right and left, respectively, of the center of a cross section perpendicular to the axis of the venturi.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to exhaust gas purification apparatus for internal combustion engines, and more particularly, to techniques of injecting a reducing agent into an exhaust pipe to supply the reducing agent to an exhaust gas purification catalyst.

2. Description of the Related Art

An SCR (Selective Catalytic Reduction) system has been known as an exhaust gas aftertreatment system for purifying the exhaust gas emitted from a diesel engine. Such an SCR system includes an injection nozzle arranged upstream of an SCR catalyst inserted in an exhaust pipe of the engine, and a reducing agent is injected from the injection nozzle to be supplied to the SCR catalyst, which then reduces the nitrogen oxides contained in the exhaust gas to harmless nitrogen and water. Further, the exhaust pipe has a throat located near the reducing agent injecting position. The throat serves to increase the flow velocity of the exhaust gas and thus to allow the reducing agent to be satisfactorily diffused into the exhaust gas, thereby improving the exhaust gas purification performance of the SCR catalyst (Unexamined Japanese Patent Publication No. 2002-213233).

As the reducing agent for the SCR system, an aqueous solution of urea is widely used in view of safety and ease of handling.

Generally, the throat of the exhaust pipe is obtained by preparing two press-molded parts formed using respective two axially divided molds and joining the parts together by welding or the like.

In the case of the exhaust pipe obtained by joining parts together, however, the aqueous urea solution injected from the injection nozzle is liable to collect on that joint of the exhaust pipe which is located at a lower level, creating the possibility of urea being separated from the aqueous urea solution and deposited on the joint. If urea deposits on the joint, the deposit tends to spread from there, so that urea is likely to deposit inside the throat. Also, if urea is deposited on the joint, the exhaust pipe may possibly start to corrode from the joint. Further, if urea, which is the constituent of the aqueous urea solution injected from the injection nozzle, separates out inside the throat, then the amount of urea diffused into the exhaust gas decreases, giving rise to a problem that the exhaust gas purification performance of the SCR catalyst lowers.

SUMMARY OF THE INVENTION

The present invention was made to solve the above problems, and an object thereof is to provide an exhaust gas purification apparatus for an internal combustion engine which apparatus is capable of preventing a reducing agent from separating out at joints of an exhaust pipe, thereby restraining deposition of the reducing agent inside a throat of the exhaust pipe.

To achieve the object, the present invention provides an exhaust gas purification apparatus for an internal combustion engine, comprising: an exhaust gas purification catalyst arranged in an exhaust pipe of the engine; a throat located upstream of the exhaust gas purification catalyst and formed by narrowing part of the exhaust pipe; and a reducing agent injection device located at the throat or upstream of the throat close thereto, for injecting a reducing agent into the exhaust pipe, characterized in that the throat is formed by joining two axially divided parts together and is positioned such that joints thereof are located to the right and left, respectively, of a center of a cross section perpendicular to an axis of the throat.

In the exhaust gas purification apparatus of the present invention, neither of the joints is situated at a lower part of the throat, and thus, the reducing agent injected from an injection nozzle of the reducing agent injection device does not collect on the joints. Consequently, the reducing agent does not separate out at the joints, preventing deposit of the reducing agent from spreading from the joints and thereby restraining deposition of the reducing agent inside the throat.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from the detailed description given hereinafter and the accompanying drawings which are given by way of illustration only, and thus, are not limitative of the present invention, and wherein:

FIG. 1 schematically illustrates an exhaust system of an internal combustion engine according to the present invention;

FIG. 2 is a side view of a venturi;

FIG. 3 is a cross-sectional view of the venturi taken along a plane perpendicular to the axis of the venturi; and

FIG. 4 illustrates an example of how urea is deposited inside the venturi.

DETAILED DESCRIPTION OF THE INVENTION

A preferred embodiment of the present invention will be described below with reference to the accompanying drawings.

FIG. 1 shows a schematic construction of an exhaust system of a diesel engine (hereinafter merely referred to as the engine 1) to which an exhaust gas purification apparatus of the present invention is applied.

The engine 1 is mounted on a motor vehicle and serves as a driving power source for moving the vehicle.

As shown in FIG. 1, the exhaust system of the engine 1 employs an SCR system 2 for reducing and thereby removing nitrogen oxides contained in the exhaust gas. The SCR system 2 comprises an SCR catalyst 4 inserted in an exhaust pipe 3 of the engine 1, and a reducing agent supply device 5 for supplying an aqueous solution of urea, as a reducing agent, to the inside of the exhaust pipe 3, that is, the exhaust gas.

The reducing agent supply device 5 includes a tank 6 storing the aqueous urea solution, an injection nozzle 7 having a distal end opening into the exhaust pipe 3 at a location upstream of the SCR catalyst 4, and a pump 8 for feeding the aqueous urea solution in the tank 6 to the injection nozzle 7. The operation of the pump 8 is controlled in accordance with operating conditions of the engine 1 and the like so that the aqueous urea solution in the tank 6 may be injected through the injection nozzle 7 into the exhaust pipe 3.

The aqueous urea solution injected from the injection nozzle 7 of the reducing agent supply device 5 into the exhaust pipe 3 undergoes hydrolysis due to heat of the exhaust gas and the presence of water, forming ammonia as a result. The ammonia flows, together with the exhaust gas, to the SCR catalyst 4, where the ammonia reduces the nitrogen oxides contained in the exhaust gas and decomposes into harmless nitrogen and water.

A filter 9 for removing PM (Particulate Matter) and the like in the exhaust gas is arranged upstream of the SCR catalyst 4, to prevent the PM and the like from flowing to the SCR catalyst 4. The exhaust pipe 3 has a venturi (throat) 10 located between the filter 9 and the SCR catalyst 4. The venturi 10 narrows, or constricts, the flow passage of the exhaust pipe 3 to thereby increase the flow velocity of the exhaust gas.

The injection nozzle 7 of the reducing agent supply device 5 is positioned in the venturi 10 such that the injection opening thereof is located substantially at the center of a cross section perpendicular to the axis of the venturi 10. Namely, the aqueous urea solution is injected into the exhaust gas of which the flow velocity is increased by the venturi 10. By injecting the aqueous urea solution into the exhaust gas flowing at an increased speed, it is possible to cause the aqueous urea solution to be satisfactorily diffused into the exhaust gas, thus improving the efficiency of the hydrolysis as well as of the reduction reaction of the SCR catalyst 4.

Further, a mixing/diffusing device 11 is arranged immediately upstream of the injection nozzle 7. The mixing/diffusing device 11 has the function of producing a swirling flow of the exhaust gas, for example, and serves to further the aqueous urea diffusion effect of the venturi 10.

FIGS. 2 and 3 illustrate the construction of the venturi 10 in detail, wherein FIG. 2 is a side view of the venturi and FIG. 3 is a sectional view of the venturi taken along a plane perpendicular to the axis of the venturi.

As shown in FIG. 2, the venturi 10 has a shape resembling an hourglass formed by constricting a central portion of a tubular member. The venturi 10 with such a shape is obtained by joining axially divided upper and lower half members 10 a and 10 b together by welding or the like. Each of the upper and lower members 10 a and 10 b is formed by pressing and has a smooth inner wall surface. Also, as shown in FIG. 3, the venturi 10 is arranged in the vehicle such that joints 10 c of the upper and lower members 10 a and 10 b are located just to the right and left, respectively, of the center C of a cross section perpendicular to the axis of the venturi 10.

In the exhaust gas purification apparatus of the present invention constructed as stated above, even though the aqueous urea solution injected from the injection nozzle 7 adheres to the inner wall of the venturi 10, the solution does not collect on the joints 10 c, because neither of the joints 10 c is located at the lower part of the inner wall. Accordingly, urea is prevented from separating from the aqueous urea solution at the joints 10 c, restraining deposition of urea inside the venturi 10. Thus, urea is prevented from depositing on the joints 10 c, whereby corrosion of the joints 10 c is prevented. Also, since deposition of urea inside the venturi 10 is restrained, the aqueous urea solution injected from the injection nozzle 7 is diffused in full into the exhaust gas, making it possible to ensure the required exhaust gas purification performance of the SCR catalyst 4.

FIG. 4 illustrates an example of how urea is deposited inside the venturi 10.

More specifically, FIG. 4 shows how urea is deposited inside the venturi 10 when the engine is operated at a low speed over a long period of time.

Where low-speed engine operation such as idling is repeated over a long period of time, it is likely that urea separates from the aqueous urea solution at the bottom 10 d of the venturi 10 and deposits there. However, urea, if deposited, accumulates on the lower part of the venturi 10. It was ascertained by experiment that no urea was deposited on an inner wall region of the venturi 10 at and above an angular range which was 22.5° or more below a horizontal line Lh passing through the center C of the venturi 10 where the aqueous urea solution was injected, as shown in FIG. 4. Accordingly, by positioning the venturi 10 such that the joints 10 c thereof are each located within an angular range between 22.5° above and 22.5° below the horizontal line Lh passing through the center C, it is possible to prevent urea from depositing on the joints 10 c without fail.

The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims. 

1. An exhaust gas purification apparatus for an internal combustion engine, comprising: an exhaust gas purification catalyst arranged in an exhaust pipe of the engine; a throat located upstream of the exhaust gas purification catalyst and formed by narrowing part of the exhaust pipe; and a reducing agent injection device located at the throat or upstream of the throat close thereto, for injecting a reducing agent into the exhaust pipe, wherein the throat is formed by joining two axially divided parts together and is positioned such that joints thereof are located to the right and left, respectively, of a center of a cross section perpendicular to an axis of the throat.
 2. The exhaust gas purification apparatus according to claim 1, wherein the throat is positioned such that the joints are each located within an angular range between 22.5° above and 22.5° below a horizontal line passing through the center of the cross section.
 3. The exhaust gas purification apparatus according to claim 1, wherein an injection nozzle of the reducing agent injection device has an injection opening located substantially at the center of the cross section.
 4. The exhaust gas purification apparatus according to claim 3, further comprising a mixing/diffusing device arranged immediately upstream of the injection nozzle. 